Stimuli Responsive Silylene: Electromerism Induced Reversible Switching Between Mono‐ and Bis‐Silylene

Abstract Electromerism is a very well‐known phenomenon in transition metal chemistry. In main group chemistry, this concept has only started getting attention recently. We report stimuli responsive low‐valent silicon compounds exhibiting electromerism. A mixed‐valent silaiminyl‐silylene 1, [LSi−Si(NDipp)L] (L=PhC(N t Bu)2), was synthesized in a single step from amidinate‐chlorosilylene. Compound 1 has two interconnected Si atoms in formally +I and +III oxidation states. Upon treatment with Lewis acidic CuIX (X=mesityl, Cl, Br, I), electron redistribution occurs resulting in the formation of [{LSi(NDipp)Si(L)}−CuX], in which both silicon atoms are in the +II formal oxidation state. Removal of the copper center from [{LSi(NDipp)Si(L)}−CuX] by using a Lewis basic carbene led to reformation of the precursor [LSi−Si(NDipp)L]. Thus, the process is fully reversible. This showcases the first example of Lewis acid/base‐induced reversible electromerism in silicon chemistry.


Synthesis of [{LSi-Si(L)=NDipp}] (1)
To a mixture of [LSiCl] (5 g, 16.96 mmol) and [DippN(H)Li] (3.11 g, 16.96 mmol) ice-cold 100 mL of toluene was added slowly while stirring. The reaction mixture was stirred at room temperature for 18 h. The reaction mixture was filtered through P4 frit and the residue was washed with 20 mL toluene. All the volatiles were removed from the filtrate. The product was washed 20*3 mL pentane and dried in vacuo to obtain a yellow solid.

Synthesis of [{LSi(NDipp)Si(L)}-CuMes] (2a)
Toluene (10 mL) was added to a mixture of 1 (500 mg, 0.72 mmol) and mesityl-copper (131 mg, 0.72 mmol) at room temperature. The reaction mixture immediately turned dark red. The reaction was further stirred for 3 hours at room temperature and then all the volatiles were removed. The red solid was dissolved in minimum amount of pentane and stored at -30 ⁰C. After few days dark red-coloured crystals were obtained which were suitable for x-ray diffraction studies. The motherliquor was decanted off and the product was dried in vacuo.  To a mixture of 1 (500 mg, 0.72 mmol) and CuX (X = Cl, 71 mg, 0.72 mmol; X = I, 137 mg, 0.72 mmol) 10 mL of toluene was added, and the suspension was stirred for 18 hours at room temperature. Then the reaction mixture was heated to obtain a clear red solution. The solution was allowed to stand at room temperature to obtain red-coloured crystals suitable for x-ray diffraction analyses. The mother-liquor was decanted off and the product was dried in-vacuo.

Synthesis of [{LSi(NDipp)Si(L)}-CuBr] (2c)
To a mixture of 1 (500 mg, 0.72 mmol) and CuBr (103 mg, 0.72 mmol) 10 mL of thf was added, and stirred for 18 hours at room temperature. The reaction mixture was concentrated till incipient crystallization and then stored at -30 ⁰C. After few days red coloured crystals were collected by decanting off the mother-liquor and subsequently dried in-vacuo.

Single crystal X-ray diffraction analysis
A suitable crystal was covered in mineral oil (Aldrich) and mounted on a glass fiber. The crystal was transferred directly to the cold stream of a STOE IPDS 2 or a STOE StadiVari diffractometer.
All structures were solved by using the program SHELXS/T [4] and Olex2. [5] The remaining nonhydrogen atoms were located from successive difference Fourier map calculations. The refinements were carried out by using full-matrix least-squares techniques on F2 by using the program SHELXL.

Comment to compound 2c:
After completion of the structure refinement, two relatively large residual electron densities remain, which can be assigned to a Cu-Br fragment with identical bond length to the actual target molecule. The chemical occupation is about 4%. This suggests that these are atom positions of an unresolved twin molecule, in which the positions of the lighter remaining atoms cannot be assigned due to too low electron density and overlaps. This not obvious twinning was overlooked during the measurement of the crystal, but also afterwards a twin integration and the search for a suitable twin law fails.

Quantum chemical calculations
To investigate the bonding properties in the system under discussion [LSi-Si(NDipp)L] 1, [LSi-Si(NPh)L], 2d, CuI, ITMe and ITMe-CuI, quantum chemical RI-DFT calculations were performed using the BP-86 functional. [6] The basis sets were of def-SVP quality for all atoms as given in the program package TURBOMOLE. [7] For iodine an effective core potential (ecp) of 46 core electrons was chosen. Calculations were performed under the constraint of symmetry C2 for the (bis) silylene compounds and complex 2d, otherwise without symmetry constraints. The reaction progress between 1 and II were calculated in a preliminary way using the same calculation method ( Figure S39).
The Si NMR shifts of 1, II and 2d were calculated using the mpshift module [8] of the TURBOMOLE program package. Tetramethylsilane (TMS) was taken as a reference substance in the calculation. Partial charges were determined using the Ahlrichs-Heinzmann population analysis based on occupation numbers. [9] Contour plots of the electronic charge density and bond critical points were obtained using the program multiwfn. [10] The transformation pathway for the isomerization of II to 1 was pre-optimized with the corresponding tool in TURBOMOLE, a chain-of-states method that optimizes reaction paths under the sole constraint of equally spaced structures [11] by employing 79 intermediate structures.
The resulting pathway is shown in Fig. S39 and also available as movie in file path.mp4. Starting from II, at first NDipp moves via TS1 from the middle of the Si-Si bond to one of the Si atoms (LM1), then the L unit at the other Si rotates to give 1. Final optimizations of stationary points were done with a fine grid (grid 5) [12] and employing weight derivatives, as well as with auxiliary basis sets of higher quality [13] and a tight self-consistent field (SCF) threshold of 10 -9 Eh for the energy and 10 -4 for the gradient norm in the structure optimizations. This yields the relative energies shown in Table S2. For II/LM1/1 the lowest frequencies amount to 20/16/14 cm -1 , for TS1, one imaginary frequency was found amounting to 102i cm -1 . The second part of the transformation exhibits rather a plateau than a sharp transition state due to the coupled movement of the Si-N-C-N ring and the ligands, see Fig. S39. Even by thorough optimization it was not possible to achieve a gradient norm below 4*10 -4 and also not possible to separate the corresponding two modes with low imaginary frequencies. The mode dominated by the rotation of the Si-N-C-N ring amounts to 14i cm -1 , the one rather dominated by rotations within the ligands to 8i cm -1 . The energy for this state, TS2, is lower than that for TS1. Thus, surmounting the latter is the determining step. For the optimized structures thermal contributions for T=298.15K were calculated within the harmonic oscillator rigid rotor model [14] with frequencies scaled by 0.9914. Further, for these structures single point calculations with the same strict settings were done with B3LYP [15] /def2-TZVP [16] without and with COSMO [17] for modelling the solvent (epsilon=2.4 for toluene), and with the D3 [18] correction for dispersive interactions.